1. Field of the Invention
The present invention relates to an optical head for reproducing or recording information on optical information storage media, an apparatus for reproducing and/or recording optical information, and their manufacturing method.
2. Background Art
Optical disc units for CDs and DVDs are widely available examples of optical information reproducing apparatuses. In CDs, light with the wavelength of 780 nm is focused beyond a 1.2-mm thick substrate by an optical head with the numerical aperture (NA) of 0.45. In DVDs, the wavelength is reduced to 650 nm, and the NA is increased to 0.6 so as to achieve higher capacities than CDs. As a result, the thickness of the substrate of the DVD is set to be 0.6 mm in order to reduce the influence of coma aberration that is produced when the disc is inclined. In recent years, large-capacity optical discs referred to as Blu-ray Discs (BDs) have also been put on the market, in which a blue-violet laser diode is used. In BDs, the NA of the objective lens is increased to 0.85 for even greater capacities. At the same time, in order to reduce the influence of the tilting of the disc, the thickness of the substrate is reduced to 0.1 mm. In practice, however, a 0.1-mm thick substrate is unable to carry a 120-mm disc. Therefore, a 0.1-mm thick cover layer is provided on a 1.1-mm thick substrate, and light is focused beyond the cover layer.
An example of an optical head used in such disc systems is disclosed in Patent Document 1 (JP Patent Publication (Kokai) No. 11-144297 A (1999). In this example, a semiconductor laser chip is integrally formed with a prism, a photodetector, and a substrate, and one such unit is stacked on top of the other in two stages so as to handle the two kinds of optical discs, namely CDs and DVDs, for example. Light emitted from such a module is focused on a disc by an objective lens separately mounted on an actuator. The light is then reflected back to the same module, where it is reflected in the prism and then received by the photodetector.
Another conventional example of an optical head is disclosed in Patent Document-2 (JP Patent Publication (Kokai) No. 2004-103241 A), in which a semiconductor laser, a prism, and a photodetector are also combined into a module. Light emitted by the module is also focused by an externally disposed objective lens onto an optical disc and then returned back to the module. This example differs from that of Patent Document 1 in that a diffraction grating is added to the module, whereby the reflected light from the optical disc is guided to the photodetector.
In yet another example of an optical head, Patent Document 3 (JP Patent Publication (Kokai) No. 2004-272951 A) discloses a module consisting of a semiconductor laser and a photodetector. The module is further integrated with an objective lens as well as a diffraction grating, which is disposed in an upright manner inside the module such that it can act on the light from the semiconductor laser before it is reflected by a mirror. In an optical disc unit based on this technology, the optical head is mounted on a swing-arm actuator so that the entire optical head can be actuated for reproducing a signal from the optical disc.
Patent Document 4 (JP Patent Publication (Kokai) No. 6-251410 A (1994), corresponding to U.S. Pat. No. 5,481,386) discloses yet another example of an optical head in which a surface-emission laser, a photodetector, a diffraction lens, and a diffraction grating are integrally fabricated in a module. The light emitted from the surface-emission laser is focused on an optical disc by a diffractive lens, and the reflected light is guided to the photodetector by the diffraction grating. In an optical disc unit based on this technology, the optical head is disposed on a swing arm so that the entire optical head can be actuated for positioning a light spot on a particular information track.
Patent Document 1: JP Patent Publication (Kokai) No. 11-144297 A (1999)
Patent Document 2: JP Patent Publication (Kokai) No. 2004-103241 A
Patent Document 3: JP Patent Publication (Kokai) No. 2004-272951 A
Patent Document 4: JP Patent Publication (Kokai) No. 6-251410 A (1994)